The present invention relates to an alignment detection optical system, or more in particular to an exposure system of reduced projection type comprising an alignment detection optical system of TTL (Through The Lens) type for detecting marks on a wafer through a projection optical system.
In conventional methods of fabricating a semiconductor device, a stepper is used for projection exposure of a circuit pattern on a mask while moving the wafer stepwise. A semiconductor device is fabricated by sequential alignment and superimposed exposure of a circuit pattern on a mask and a circuit pattern on a wafer coated with a resist providing a photosensitive material. This alignment is effected by detecting alignment marks on the wafer. A high-precision alignment requires a TTL system for detecting the marks through an exposure projection detection system as an effective means. The resist is coated while rotating the wafer, and therefore, as shown in FIG. 4, an asymmetric thickness distribution of a resist film 312 is caused by resist flow in the direction of detection in the vicinity of an uneven base mark 311. When the resist is irradiated with a light of narrow wavelength range the detected light intensity changes periodically with the resist film thickness as shown in FIG. 5A. As a result, the image detection of an alignment mark having an asymmetric resist distribution with a light of narrow wavelength range causes an increased emphasis of asymmetry of the resist film thickness distribution resulting in a detection error of the detected wavelength as shown in FIG. 5B. The irradiation of a light of broad wavelength range, on the other hand, reduces the change in the intensity of the detected light with the resist film thickness as shown in FIG. 6A, with the result that the detected waveform becomes symmetric thereby making possible a high-accuracy alignment as shown in FIG. 6B.
The compatibility between the TTL system and the detection with a light of broad wavelength range requires the resolution of the problem mentioned below.
In a projection optical system, the aberration is corrected generally only for the exposure light of single wavelength for transferring a fine pattern. The detection by the TTL system with a light of broad wavelength range, therefore, displaces the image points of various wavelengths. The resulting chromatic aberration blurs the detection waveform and makes a high-accuracy alignment impossible. In order to obviate this problem, as disclosed in JP-A-1-227431, a method has been suggested in which an optical system for correcting the chromatic aberration for a light of broad wavelength range is configured of glass lenses and inserted between a projection lens and a detector.
In the prior art described above, an optical system for chromatic aberration correction is configured of glass lenses, and therefore an increased number of glass lenses are used for the chromatic aberration correction system, thereby undesirably reducing the resolution of the alignment detection optical system as a whole, due to the adjustment error or the surface error of the optical elements.